repaint是强制刷新，不管死活的，update是放入事件队列，让qt事件机制去处理。repaint频率过高很容易崩掉的。可自行看qt帮助文档！ (2017-10-30 17:12)
Repaints the widget directly by calling paintEvent() immediately, unless updates are disabled or the widget is hidden.
We suggest only using repaint() if you need an immediate repaint, for example during animation. In almost all circumstances update() is better, as it permits Qt to optimize for speed and minimize flicker.
Warning: If you call repaint() in a function which may itself be called from paintEvent(), you may get infinite recursion. The update() function never causes recursion.http://doc.qt.io/qt-5/qwidget.html#repaint